In the aircraft and aerospace industries, aluminum alloys are used extensively because of the durability of the alloys as well as a reduction in weight achieved by their use. Alloys in aircraft and aerospace industries must have excellent strength and elongation properties and superior exfoliation resistance and fracture toughness. A number of aluminum alloys have been developed for these industries to satisfy these needs. However, and in view of the continuing demands of the industry for weight reduction, increased strength to weight ratio requirements and improved performance in corrosive climatic conditions, a need has developed for an aluminum-based alloy having superior fracture toughness and exfoliation resistance. The present invention meets this need in the aircraft and aerospace industries by providing an aluminum-zinc-magnesium-copper alloy which contains controlled and stoichiometric amounts of copper, magnesium and zinc.
Aluminum alloys are known in the art which contain zinc, magnesium and copper. In particular, AA 7000 series have been developed for particular use in aircraft and aerospace applications. AA 7150, as registered with the Aluminum Association, includes 1.9-2.5% by weight of copper, 2.0-2.7% by weight of magnesium and 5.9-6.9% by weight of zinc, 0.08-0.15% by weight of zirconium, a maximum of 0.12% by weight of silicon, a maximum of 0.15% by weight of iron, with the remainder being aluminum and other inevitable impurities.
For these types of aluminum alloys, adjustments have been proposed in both composition and processing variables to achieve improved strength and corrosion properties. U.S. Pat. No. 3,881,966 to Staley et al. discloses an aluminum based alloy containing zinc, copper and magnesium, together with zirconium, which exhibits very high strength when thermally treated to a condition having high resistance to stress corrosion cracking. A special aging treatment produces the optimum combination of strength and resistance to stress corrosion cracking.
U.S. Pat. No. 4,305,763 to Quist et al. discloses a 7000 series aluminum alloy characterized by high strength, high fatigue resistance and high fracture toughness. This combination of properties is achieved by controlling the chemical composition ranges of the alloying and trace elements, by heat treating the alloy to increase its strength to high levels, and by maintaining a substantially unrecrystallized microstructure.
U.S. Pat. No. 4,828,631 to Ponchel et al. is drawn to an improved high strength 7000 series aluminum alloy having specific and controlled amounts of alloying constituents that is produced using isothermal aging in a single step process. This alloy develops improved resistance to exfoliation by aging at a temperature from about 270.degree. F. to about 285.degree. F. for a period of from 6-30 hours or 6-60 hours.
However, a need still exists for AA 7000 series aluminum-based alloys which have superior exfoliation corrosion resistance and fracture toughness without sacrificing strength and/or elongation.
The present invention is directed to a method of producing an improved aluminum-based product having superior exfoliation resistance and fracture toughness. The method of the present invention includes providing an aluminum-based alloy having controlled alloying components as described herein which, when processed according to the method of the invention, has outstanding exfoliation corrosion resistance and fracture toughness.